The present invention relates to antennas and particularly relates to microstrip antennas used to receive global positioning data from satellites.
The United States Government has placed into orbit a number of satellites as part of a global positioning system (GPS). A GPS receiver gets signals from several GPS satellites and can very accurately determine certain parameters, such as position, velocity, and time. There are both military and commercial uses for GPS systems. A primary military use is in aircrafts or ships to constantly determine the position and velocity of a plane or a ship. An example of a commercial use includes surveying and the accurate determination of a fixed point location or the difference between two fixed points, with a high degree of accuracy. Another example is a generation of a high-accuracy timing reference.
Each satellite continually transmits two L-band signals. A receiver simultaneously detects signals from several satellites and processes them to extract information from the signals in order to calculate desired parameters such as, for example, position, velocity or time. The United States Government has adopted standards for these satellite transmissions so that others may use the satellite signals by designing receivers for specific purposes. The satellite transmission standards are set forth by an "Interface Control Document" of Rockwell International Corporation, entitled "NAVSTAR GPS Segment/Navigation User Interfaces", dated Sep. 26, 1994, as revised on Dec. 19, 1996.
Each satellite transmits an L1 signal on 1575.42 MHz carrier. A second, L2 signal is transmitted by each satellite, having a carrier frequency of 1227.6 MHz. Both signals are modulated in the satellite by a pseudo-random signal function that is unique to that satellite. This results in a spread-spectrum signal that resists radio-frequency noise or an intentional jamming. It also allows the L-band signals from a number of satellites to be individually identified and separated in the receiver. One pseudo-random function is the precision code (P-code), it modulates both of the L1 and L2 carriers in the satellite. The P-code has a 10.23 MHz clock rate and thus causes the L1 and L2 signals to have a 20.46 MHz bandwidth. The length of the code is seven days; that is, the P-code pattern begins again every seven days. The L1 signal of each satellite is also modulated by a second pseudo-random function or unique clear acquisition code (C/A code) having a 1.023 MHz clock rate and repeating its pattern once every millisecond. Further, the L1 carrier is modulated by a 50 bit-per-second navigational data stream which provides certain information of satellite identification, status and the like.
In the receiver, the process of demodulating the satellite signals corresponding to the known pseudo-random functions are generated and aligned in phase with those modulated onto the satellite signals. The phase of the carriers from each of the satellites being tracked is measured from the result of correlating each satellite signal with a locally generated pseudo-random function. The relative phase of the carrier signals from a number of satellites is a measurement that is used by a receiver to calculate the desired end values of distance, velocity, time, etc. Since the P-code encrypted functions are classified by the U.S. Government so that they can be used for military purposes only, commercial users of the GPS must work directly only with the C/A code pseudo-random function.
The Government of the former U.S.S.R. has placed into orbit a similar satellite positioning system called "GLONASS"; more information on the standard can be found in the "Global Satellite Navigation System GLONASS--Interface Control Document" of the RTCA Paper No. 518-91/SC159-317, approved by the Glavkosmos Institute of Space Device Engineering, the official former U.S.S.R. GLONASS responsible organization. The GLONASS device has L1 carrier frequencies in the range of 1602-1616 MHz.
Devices receiving the global satellite positioning signal typically use microstrip patch antennas. The antennas are designed to strongly receive the energy in the wavelength range transmitted by the satellites. In many examples, the antennas are designed to receive a narrow bandwidth of the right-hand circular polarized waves of a certain band such as the L1 band. One example of a microstrip antenna uses a rectangular patch region positioned on a dielectric substrate. The length and width of the rectangular region are chosen in order to receive a narrow bandwidth about the L1 bands.
A microstrip patch antenna is characterized by a narrow operating frequency band, and precautions must be taken to keep the required values of the gain, the axial ratio and the voltage standing wave ratio (VSWR) for the signals over the desired bandwidth. This is especially difficult when the L1 frequency bands of both GPS and GLONASS satellites are detected. It is desired to increase the bandwidth of antenna in order that the full L1 frequency band from both the GPS and the GLONASS devices can be received.
An additional problem with GPS and GLONASS antennas concerns multipath interference. One of the major factors influencing the final accuracy of measurements of the distance, velocity, etc., is the accuracy of the signal phase measurements. This phase measurement precision is altered, if in addition to the direct line-of-sight propagation signal, a multipath fading signal is also received. For this reason, it is desired to have an antenna that reduces the multipath signals received.